In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art.
The use of oxide ceramics as a framework material for dental restorations has long been state of the art. This material is characterized by an excellent biocompatibility and outstanding mechanical properties. For many years it has also been widely used as an implant material and for prostheses. In the past few years, ceramics based on partially stabilized ZrO2 ceramics have been used in particular.
The shaping of these ceramics in dental engineering is typically performed by mechanical means. In particular, milling of partially sintered ceramics with CAD/CAM processing units has gained acceptance. The shrinkage which occurs during final densification of shaped bodies, going from a density of approximately 40-60% to a density of more than 95%, is taken into account during the mechanical processing. The quoted density is relative to the theoretical density.
The disadvantages of ZrO2 ceramics are the low translucency and their milky-white color. A non-colored and non-coated restoration or restoration part looks like an unnatural tooth. Coloring the ZrO2 ceramic to match the patient's situation for an aesthetic tooth reconstruction are thus essential.
A particularly great disadvantage of sintered ceramics according to the state of the art is that they do not produce blanks for CAD/CAM processing in open-pored form or in dense-sintered form which are multi-colored or have zones of different colors corresponding to the coloration of a natural tooth.
All that is known from the state of the art is a series of technical solutions for colored, not multi-colored, blanks. However, these solutions have the disadvantage that the natural tooth color, the color gradient, the polychromatism, the graduated translucency and brightness of color were not achieved. These known solutions are described as follows:
The preparation of an open-porous colored and white Y2O3-containing ZrO2 blank is achieved according to EP 1 210 054 from liquids via co-precipitation from chlorides which contain Zr, Y, Al, Ga, Ge, In, Fe, Er and Mn ions. By means of the co-precipitation and subsequent calcination, the prepared powder already contains the coloring ions before shaping. Oxides from the group Fe2O3, Er2O3 and MnO2 are selected as coloring compounds. The disadvantage of this approach is that a very costly and laborious method of the co-precipitation process with subsequent calcination must be carried out in order to obtain a colored powder. This means that this process must be carried out for every single color.
In the following disclosures monolithic ceramics are presented which each allow one specific color, thus polychromatism, is not achieved.
U.S. Pat. No. 5,263,858 (Yoshida et el.) describes the preparation of ivory-colored shaped bodies for dental applications (brackets), wherein during the preparation of the stabilized ZrO2 ceramic coloring compounds in solutions are added before the calcination, or as powdery mixtures of coloring oxides after the calcination. In order to achieve the desired ivory shade, the addition of Fe2O3, Pr6O11 and Er2O3 is necessary. However, this process has the disadvantage that it is a multi-stage process.
It is further known from the state of the art according to FR 2 781 366 to mix the coloring components with the starting powder of the ZrO2, grind and sinter jointly. Fe2O3, CeO2 and Bi2O3 are mentioned as coloring oxides.
EP 0 955 267 mentions contents of 5-49 wt.-% CeO2, whereby coloration is achieved.
For the preparation of completely cubically stabilized zirconium dioxide in an arc-furnace process, according to EP 1 076 036 B1 one or more stabilizing and coloring oxides or their precursors are added to a ZrO2 source. The coloring oxides of the elements Pr, Ce, Sm, Cd, Tb are inserted into the crystal lattice of the ZrO2 after the sintering process.
U.S. Pat. No. 5,656,564 relates to the preparation of zirconium oxide shaped bodies which contain oxides of the rare earths boron oxide, aluminum oxide and/or silicon oxide. The shaped bodies contain zirconium dioxide as a mixed phase of tetragonal and monoclonal ZrO2. Oxides of the elements Pr, Er and Yb are introduced into the sintered ceramic as coloring oxides.
Technical solutions are further known according to the state of the art which allow colored blanks to be obtained by infiltration of liquids. However, these technical solutions have the serious drawback that coloring takes place after the pre-sintering process, and thus liquids are introduced into an open-porous ceramic body. The coloring is not completely homogeneous and also a multi-coloration cannot be achieved.
Unlike sintered ceramics, such as ZrO2 and Al2O3, a process for the preparation of multi-colored glass ceramic blanks is known from DE 197 14 178 C2. However, the preparation of multi-colored ZrO2 blanks is not mentioned in this document.
A disadvantage of the state of the art is that multi-colored sintered ceramic blanks cannot be prepared. Moreover, the solutions according to the state of the art are very costly and quality problems arise. The latter applies to the infiltration technique. Due to the subsequent coloring of a partially sintered blank or of a shaped dental product only the voids (pores) between the partially sintered particles of the starting powder can be occupied by the coloring ions. As a result, only discrete areas of the surface of the particles are colored with a layer of the coloring oxides, a continuous coverage of the surface of the particles of the starting powder not being possible. A further great disadvantage with infiltration is the concentration gradient of the coloring from the outside inwards. If a porous body is introduced into the coloring solution, the starting solution releases part of the dissolved coloring ions, starting from the outside inwards, and thus the coloring solution is “depleted” of some of its coloring substances. The consequence of this is that there is a higher concentration of the coloring ions on oxides outside than in the inside of the shaped body. Furthermore, only a certain depth of penetration can be achieved by means of the infiltration technique.